1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display DEVICE which has organic EL elements and thin film transistors (hereinafter referred to as TFTs) acting as switching elements.
2. Description of Related Art
FIG. 1 is a plan view illustrating the display pixel section of a conventional electroluminescence display. FIG. 2A is a cross sectional view illustrating the EL display taken along the line Axe2x80x94A of the FIG. 1. FIG. 2B is a cross sectional view illustrating the EL display taken along the line Bxe2x80x94B of FIG. 1.
Referring to FIG. 1, the area, surrounded by gate signal lines 51 and drain signal lines 52, defines a display pixel. A first thin film transistor 30 is formed adjacent to the intersection of the signal lines. A capacitor is formed between the source 13s of the thin film transistor 30 and a storage capacitor electrode line 54. The source 13s of the thin film transistor 30 acts as a capacitor electrode 55 and is connected to the gate 41 of a second thin film transistor 40. The second thin film transistor 40 has the source 43s connected to the anode 61 of an organic EL element 60 and the drain 63d connected to a power source line 53 for driving the organic EL element.
The storage capacitor electrode line 54 is disposed in parallel to the gate signal line near the thin film transistor. The storage capacitor electrode line 54 is formed of, for instance, chromium. A capacitor is formed between the storage capacitor electrode line 54 and the capacitor electrode 55 connected to the source 13s of the thin film transistor, sandwiching the gate insulating film 12, to accumulate charges. This storage capacitor holds the voltage applied to the gate 41 of the second thin film transistor 40.
In order to fabricate an organic EL display, display pixels, which each include the organic EL element 60 and the thin film transistors 30 and 40, are arranged in a matrix form on the substrate 10.
Referring to FIGS. 2A and 2B, an organic EL display is fabricated by forming, in order, thin film transistors and organic EL elements on a glass or resin substrate 10 or a conductive or on a semiconductor substrate 10. When the conductive or semiconductor substrate is used as the substrate 10, an insulating film of Sio2 or SiN is formed on the substrate 10. Then, the thin film transistors 30 and 40 and the organic EL display element 60 are formed over the insulating film.
First, the first thin film transistor 30 being a switching thin film transistor will be explained below.
Referring to FIG. 2A, the gate signal line 51 acting as the gate electrode 11 is formed of a high melting point metal (refractory metal) of chromium (Cr), molybdenum (Mo), or the like deposited on an insulating substrate 10 of quartz glass, non-alkali glass, or the like. The drain signal line 52 is formed of aluminum (Al) on the insulating substrate. The power source line 53, which acts as a drive power source for the organic EL element and is formed of aluminum, is disposed over the insulating substrate 10.
A gate insulating film 12 and an active layer 13 of a poly-crystalline silicon (hereinafter referred to as p-Si) film are formed in order. The so-called LDD (Lightly Doped Drain) structure is formed in the active layer 13. The source 13s and the drain 13d are disposed outside the LDD regions.
An inter-layered insulating film 15 is formed all over the surface of the gate insulating film 12, the active layer 13, and the stopper insulating film 14. The inter-layered insulating film 15 is formed by depositing a Sio2 film, a SiN film, and a Sio2 film, sequentially. In order to form the drain electrode 16, the contact hole formed toward the drain 13d is filled with a metal such as aluminum. Moreover, a planarization insulating film 17 of organic resin is formed all over the surface to provide a flat surface.
Next, the second thin film transistor 60 being a drive thin film transistor for the organic EL element 60 will be explained below.
Referring to FIG. 2B, a gate electrode 41 of a refractory metal such as Cr or Mo is formed on the insulating substrate 10 of quartz glass or non-alkali glass.
The gate insulating film 12 and the active layer 43 formed of a p-Si film are deposited one after another.
In the active layer 43, an intrinsic or a substantially intrinsic channel 43c is formed above the gate electrode 41. Ions are doped into the sides of the channel 43c to form the source 43s and the drain 43d. 
A SiO2 film, a SiN film, and a SiO2 film are deposited in order all over the gate insulating film 12 and the active layer 43 to form an inter-layered insulating film 15. The contact hole formed toward the drain 43d is filled with a metal such as aluminum. Thus, a power source line 53 connected to the power source 50 is formed, a planarization insulating film 17 of organic resin is formed to flatten the whole surface. A contact hole is formed through the planarization insulating film 17 and the inter-layered insulating film 15 at the position corresponding to the source 43s. A transparent electrode, that is, the anode 61 of the organic EL element, is formed on the planarization insulating film 17. The transparent electrode is formed of an ITO (In2O3xe2x80x94SnO2 or Indium Tin Oxide) being in contact with the source 13s via the contact hole.
The organic EL element 60 has the laminated structure, which is formed by sequentially depositing an anode 61, an emissive element layer 62, and a cathode 63. The anode 61 is formed of an ITO transparent electrode. The emissive layer 62 is formed of a first hole transport layer, a second hole transport layer, an emissive layer, and an electron transport layer. The cathode 63 is formed of a magnesium indium alloy. The cathode 63 is formed on the whole surface (that is, corresponding to the whole surface of the paper) of the substrate 10 forming an organic EL display, shown in FIG. 3.
In the organic EL element, holes injected from the anode and electrons injected from the cathode are recombined inside the emissive layer. Thus, the recombination acts to excite the organic molecules included in the emissive layer to create excitons. The emissive layer emanates light when the excitons lose radiant energy. The resultant light emanates to the outside via the anode and the transparent insulating substrate.
In order to prevent a short circuit between the cathode 63 and the anode 61 caused by breakage of the emissive layer because of a step difference in thickness of the anode, the insulating film 64 (except the area surrounded by the dotted lines) is formed on the fringes of the anode 61 after formation of the anode 61. Thereafter, the emissive element layer 62 and the cathode 63 are formed. The emissive element layer 62 radiates light via the insulating substrate 10.
The organic EL element 60, as described above, is an element of the type emanating light by supplying current from the power source line 53 to the organic EL element according to the voltage applied to the gate 41 of the second thin film transistor 30. This element is a current control driven element because of the charge injection type radiation where holes injected from the anode and electrons injected from the cathode recombine inside the emissive layer to radiate light from the emissive layer. For that reason, a current density of 1 mA/cm2 is required to provide, for example, a luminance of 100 cd/cm2. A current density of 100 mA/cm2 is required to provide, for example, a luminance of 10000 cd/cm2.
Such an organic EL element requires a large current for driving. Hence, for the purpose of good displaying, the source 13s, being of a semiconductor film, and the ITO must be in reliable electric contact.
However, in the direct contact region between the source 13s, being of the p-Si film, and the ITO, a Sio2 film (an insulating film), formed on the surface of the p-Si, makes the contact unstable. As a result, there is the disadvantage that a good display quality cannot be provided.
Even if an aluminum film, with a lower resistance than the p-Si film, is formed between the source 13s and the ITO, an Al2O3 film is formed on the surface of the aluminum film so that a stable contact cannot be provided.
This invention is made to overcome the above-mentioned problems. It is an object of the present invention to provide an organic electroluminescence display that can make a reliable contact between the TFT active layer and the anode and can obtain an excellent display quality.
The above object of the present invention is achieved by an organic electroluminescence device comprising an organic electroluminescence element and a thin film transistor which are formed on a substrate; the organic electroluminescence element having at least an organic emissive layer disposed between an anode and a cathode; the thin film transistor controlling a current flowing to said organic electroluminescence element; the thin film transistor having an active layer made of a semiconductor material; and a refractory metal layer connecting a source region or drain region of the thin film transistor to the anode of said organic electroluminescence element.
In another aspect of the invention, an organic electroluminescence display device comprises pixels, each of the pixels including an organic electroluminescence element and a thin film transistor, the organic electroluminescence element having an emissive layer disposed between an anode and a cathode, the thin film transistor controlling a current flowing from a power source line to the organic electroluminescence element, the thin film transistor having an active layer made of a semiconductor material; and a contact between one of a source and drain in the active layer and the anode of the organic electroluminescence element, and between the other of the source and drain in the active layer and the power source line is respectively achieved through a refractory metal layer.
In the display device of another aspect of the invention, each pixel has a switching thin film transistor, the switching thin film transistor having a gate connected to a gate line, one of a source and drain in the active layer made of a semiconductor material and connected to a data line, and the other of the source and drain connected to a gate of a thin film transistor to control a flow of current supplied from a power source line to the organic electroluminescence element; the active layer of the switching thin film transistor making contact with the data line via a refractory metal layer.
In another aspect of the present invention, a light emitting device comprises an emissive element having an emissive layer between a first electrode and a second it electrode; a thin film transistor for controlling power supplied to the emissive element, the thin film transistor having an active layer made of a semiconductor transistor; and a refractory metal layer connecting a first electrode region in the active layer to the first electrode of the emissive element.
In the display in still another aspect of the invention, the refractory metal layer is in direct contact with the active layer made of the semiconductor material.
In the device in another aspect of the invention, the refractory metal layer comprises a first refractory metal layer which is in direct contact with the active layer made of the semiconductor material; and a second refractory metal layer which is in direct contact with the anode of the organic electroluminescence element.
Moreover, in the device in another aspect of the invention, a conductive metal layer is disposed between the first refractory metal layer and the second refractory metal layer.
In the device structure, the active layer may be formed of polycrystalline silicon. The conductive metal layer may be formed of aluminum. The anode of said organic electroluminescence element may be formed of a transparent conductive material. The first refractory metal layer may be either at least one of chromium (Cr), molybdenum (Mo), tungsten (W), and titanium (Ti), or an alloy including at least one of them. The conductive metal layer may be aluminum. Moreover, the active layer may be formed of a silicon semiconductor. The anode of the organic electroluminescence element may be indium tin oxide or indium zinc oxide.
According to the invention, the reliable contact can be formed between the active layer of the thin film transistor and the anode of an element such as an organic EL element. Hence, the thin film transistor can reliably supply power to the emissive devices such as organic EL elements so that the element can emanate stably. For that reason, the display device with the above structure can display respective pixels in a stable state and without any variation in brightness.